Air can be separated by the well-known cryogenic distillation process utilizing a thermally-linked double distillation column system to recover oxygen and nitrogen. A representative description of this well-known method is disclosed in an article by R. E. Latimer entitled "Distillation of Air" in Chemical Engineering Progress, 63 (2), 35-59 1967!. A third distillation column may be integrated with the double column system to increase the overall separation efficiency. Optionally, argon may be recovered from an intermediate sidestream in a separate argon distillation column.
In the operation of double-column cryogenic air separation systems, it is generally accepted that the amount of boilup vapor in the bottom section of the lower pressure column is a major factor in determining the oxygen recovery from feed air. If the boilup rate is reduced, oxygen recovery likewise declines. This decline in oxygen recovery with decreasing boilup is greater when the oxygen product purity is above about 98 mole %.
Several features of the double column process can reduce boilup and thereby reduce oxygen recovery. In one example, it is beneficial to withdrawn some or all of the desired nitrogen product from the higher pressure column rather than from the lower pressure column. This is advantageous and eliminates stages of nitrogen product compression, which in turn reduces capital and in some cases reduces power consumption. A disadvantage occurs, however, because as nitrogen vapor is withdrawn from the higher pressure column, boilup in the lower pressure column is reduced and oxygen recovery declines.
Refrigeration is required in the air separation process to counteract heat leak from the ambient environment and to produce some or all of the products as liquids if desired. This refrigeration typically is provided by work expansion of selected process streams. When the refrigeration demand is moderate, the refrigeration can be provided by work expanding a portion of the high pressure air feed directly into the lower pressure column. In this case, vapor flow to the higher pressure column is necessarily reduced and boilup in the bottom of the lower pressure column is also reduced, and as a result oxygen recovery declines.
Another common means of providing refrigeration is to work expand an air feed stream into the higher pressure column. In this case, it is necessary to compress the air before expansion to a pressure greater than the higher pressure column, which adds incremental power and capital costs.
Numerous ideas have been proposed in the art to increase boilup in the bottom of the lower pressure column. These ideas can be grouped into four categories: (1) raising the pressure of the stream which is to be expanded to the lower pressure column with a compressor, (2) expanding to the higher pressure column, (3) heat pumping the lower section of the lower pressure column, and (4) boiling a liquid within the process and expanding the resultant vapor.
The benefit of raising the pressure of the stream to be expanded to the lower pressure column is disclosed in German Patent DE 28 54 508 which proposes using the energy generated by the expander to drive a compressor to increase the pressure of the fluid to be expanded. This technique has the desired effect of reducing the expander flow and is commonly used in the air separation industry.
Another common practice is to expand feed air into the higher pressure column. Examples of this technique are disclosed in U.S. Pat. Nos. 5,386,691 and 5,398,514 which teach that the expansion of air to the higher pressure column has the desired effect of increasing the boilup in the lower pressure column. However, additional compression energy must be supplied to the feed. This technique increases oxygen recovery (and argon recovery if argon is a desired product) compared with expanding air to the lower pressure column, but will not necessarily reduce the power required per unit of oxygen production.
U.S. Pat. No. 5,245,831 discloses that heat pumping the bottom section of the lower pressure column will increase the oxygen recovery. This is generally attractive only if higher argon recovery also is desired. Since external compression energy must be supplied to provide recycle flow, the power required per unit of oxygen production is largely unaffected.
An alternative method of vaporizing a liquid within the process and expanding the resultant vapor, which increases argon recovery and generates increased refrigeration, is disclosed in U.S. Pat. No. 4,737,177. The higher pressure column bottoms stream is partially vaporized in an intermediate condenser of the argon column. The vaporization occurs at an intermediate pressure, and the resultant vapor is warmed and work expanded to produce refrigeration. The expanded stream provides feed to the lower pressure column. U.S. Pat. No. 5,469,710 discloses a related application of vaporizing and expanding into the lower pressure column wherein liquid is boiled at the top of the argon column to provide all the argon column reflux. The vapor which is produced by the boiling is warmed, turboexpanded to produce refrigeration, and then routed to the lower pressure column as a feed. The source of the liquid for vaporization is either the liquid from the partial or total condensation of feed air or the bottoms liquid from the higher pressure column. The benefit of increasing boilup is greater when the oxygen product purity is above about 98 mole %.
Methods which increase the boilup rate in the lower pressure column are desirable to increase oxygen recovery, especially when a high purity oxygen product is required. Methods which do not require increased compression capacity to obtain this increase in boilup are preferred. The present invention as disclosed below and defined by the claims which follow is a method which provides refrigeration for the air separation system while increasing the lower pressure column boilup rate without additional compression requirements for work expansion refrigeration.